In order to allow a semiconductor device to be high in breakdown voltage and low in loss and to be used in a high-temperature environment, silicon carbide has recently increasingly been adopted as a material forming a semiconductor device. Silicon carbide is a wide band gap semiconductor greater in band gap than silicon which has conventionally widely been used as a material forming a semiconductor device. Therefore, by adopting silicon carbide as a material forming a semiconductor device, a higher breakdown voltage and a lower on-resistance of a semiconductor device can be achieved. A semiconductor device in which silicon carbide has been adopted as a material is also advantageous in that lowering in characteristics during use in a high-temperature environment is less than in a semiconductor device in which silicon has been adopted as a material.
For example, in a metal oxide semiconductor field effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT) among semiconductor devices in which silicon carbide is employed as a material, whether a current which is to flow between two electrodes is allowed or not allowed to flow can be controlled by controlling whether or not an inversion layer is formed in a channel region with a prescribed threshold voltage being defined as a boundary.
For example, Mitsuo Okamoto et al., “Reduction of Instability in Vth of 4H—SiC Carbon Face MOSFETs,” the 59th Spring Meeting, Proceedings, The Japan Society of Applied Physics, Spring 2012, pp. 15-309 (NPD 1) has pointed out that a silicon carbide MOSFET suffers from fluctuation in threshold voltage due to gate bias stress. This document discloses a method of annealing a silicon carbide substrate having a gate oxide film formed in a hydrogen atmosphere in order to lessen fluctuation in threshold voltage.